I own a WD My Book Essential, 160GB, external hard drive and have been using it for three years with no problems. My computer's OS is Windows XP Home Edition.
I use my external as a secondary hard drive, mostly for storage space. Today, I unplugged it to take it to a friend's house. When he tried to plug the HD into his computer, his computer didn't recognize it (calling it an "unknown device"). He then tried plugging it into his laptop, which had the same result. Upon getting home, I tried plugging it back into my computer, and it did the same thing (says "Unknown Device").
Normally, whenever it's on, it's displayed on My Computer as the J: drive; ever since this happened, however, it doesn't recognize it. When I go into Device Management, it says it needs a new Driver; however, there's no driver on the WD website for the My Book Essential when run on anything above Windows 98.
I tried plugging other USB items into each of my five USB ports, and those devices worked. So, it's not a problem with any of the USB drives.